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Number And Bar Scales

6.2 Number and bar scales (EMG4Q)

Introduction to number and bar scales (EMG4R)

The two kinds of scale we will be working with in this chapter are the number scale and the bar scale. The number scale is expressed as a ratio like \(\text{1}\) : \(\text{50}\). This simply means that \(\text{1}\) unit on the map represents \(\text{50}\) units on the ground. So \(\text{1}\) \(\text{cm}\) on the map will represent \(\text{50}\) \(\text{cm}\) on the ground, or \(\text{1}\) \(\text{m}\) on the map will represent \(\text{50}\) \(\text{m}\) on the ground. To use the number scale, you need to measure a distance on a map using your ruler, and then multiply that measurement by the “real” part of the scale ratio (\(\text{50}\)) given on the map, in order to get the real distance.

The bar scale is represented like this:

Each piece or segment of the bar represents a given distance, as labelled underneath. To use the bar scale, you need to measure how long one segment of the bar is on your ruler. You must then measure the distance on the map in centimetres; calculate how many segments of the bar graph it works out to be (the total distance measured; divided by the length of one segment); and then multiply it by the scale underneath. So, if \(\text{1}\) \(\text{cm}\) on the bar represents \(\text{10}\) \(\text{m}\) on ground, and the distance you measure on the map is \(\text{3}\) \(\text{cm}\) ( \(\text{3}\) \(\text{cm}\) \(\div\) \(\text{1}\) \(\text{cm}\) length of segment = \(\text{3}\) segments) then the real distance on the ground is \(\text{3}\) \(\times\) \(\text{10}\) \(\text{m}\) = \(\text{30}\) \(\text{m}\).

Worked example 1: Using the bar and number scales

  1. You are given a map with the number scale of \(\text{1}\) : \(\text{40}\). You measure a length (on the map) of \(\text{10}\) \(\text{cm}\). What is this distance in real life?
  2. You are given a map with the number scale of \(\text{1}\) : \(\text{500}\). You measure a distance on the map of \(\text{15}\) \(\text{cm}\) with your ruler. What is this distance in real life?
  3. You are given the following bar scale: You measure the distance on the map to be \(\text{15}\) \(\text{cm}\). What is the actual distance?

  4. You are given the following bar scale: You measure the distance between two points on the map to be \(\text{11}\) \(\text{cm}\). What is the distance on the ground?

  1. Scale is \(\text{1}\) : \(\text{40}\). \(\text{10}\) \(\text{cm}\) \(\times\) \(\text{40}\) = \(\text{400}\) \(\text{cm}\) = \(\text{4}\) \(\text{m}\) The distance on the ground (in real life) is \(\text{4}\) \(\text{m}\).
  2. Scale is \(\text{1}\) : \(\text{500}\) Therefore actual distance is \(\text{15}\) \(\text{cm}\) \(\times\) \(\text{500}\) = \(\text{7 500}\) \(\text{cm}\) = \(\text{75}\) \(\text{m}\).
  3. \(\text{1}\) segment = \(\text{1,5}\) \(\text{cm}\) long, and represents \(\text{50}\) \(\text{m}\). \(\text{15}\) \(\text{cm}\) \(\div\) \(\text{1,5}\) \(\text{cm}\) (length of segment) = \(\text{10}\) so you have measured \(\text{10}\) segments in total. \(\text{10}\) segments = \(\text{10}\) \(\times\) \(\text{50}\) \(\text{m}\) = \(\text{750}\) \(\text{m}\)
  4. \(\text{1}\) segment = \(\text{2}\) \(\text{cm}\) long and represents \(\text{200}\) \(\text{m}\). \(\text{11}\) \(\text{cm}\) \(\div\) \(\text{2}\) \(\text{cm}\) (length of segment) = \(\text{5,5}\), so you have measured \(\text{5,5}\) segments in total. \(\text{5,5}\) segments = \(\text{5,5}\) \(\times\) \(\text{200}\) \(\text{m}\) = \(\text{1 100}\) \(\text{m}\) = \(\text{1,1}\) \(\text{km}\).

Using the bar and number scales

Exercise 6.1

You measure the distance between two building on a map to be of \(\text{5}\) \(\text{cm}\). If the map has a number scale of \(\text{1}\): \(\text{100}\), what is the actual distance on the ground?

\(\text{5}\) \(\text{cm}\) \(\times\) \(\text{100}\) = \(\text{500}\) \(\text{cm}\) = \(\text{5}\) \(\text{m}\)

You are given a map with the number scale \(\text{1}\) : \(\text{20}\). you measure a distance of \(\text{12}\) \(\text{cm}\) on the map. What is the actual distance in real life?

\(\text{12}\) \(\text{cm}\) \(\times\) \(\text{20}\) = \(\text{240}\) \(\text{cm}\) = \(\text{2,4}\) \(\text{m}\).

You measure a distance of \(\text{10}\) \(\text{cm}\) on a map with the following bar scale:

(\(\text{1}\) \(\text{cm}\) = \(\text{15}\) \(\text{m}\)). What is the actual distance on the ground?

\(\text{10}\) \(\text{cm}\) \(\div\) \(\text{1}\) \(\text{cm}\) = \(\text{10}\) segments. \(\text{10}\) segments \(\times\) \(\text{15}\) \(\text{m}\) = \(\text{150}\) \(\text{m}\)

You measure a distance of \(\text{15}\) \(\text{cm}\) on a map with the following bar scale:

(\(\text{2}\) \(\text{cm}\) : \(\text{100}\) \(\text{m}\)) What is the actual distance on the ground?

\(\text{15}\) \(\text{cm}\) \(\div\) \(\text{2}\) \(\text{cm}\) = \(\text{7,5}\) segments. \(\text{7,5}\) segments \(\times\) \(\text{100}\) \(\text{m}\) = \(\text{750}\) \(\text{m}\).

Using number and bar scales to measure distance (EMG4S)

In the previous section about number and bar scales we only looked at how to calculate the actual length of an object or distance between two places when we know the length we measured on the map, and the scale used. However, number and bar scales are usually seen on maps and plans. In this section, we will learn how to measure the dimensions of objects and distance on scale maps and then how to use the number and bar scale to calculate the actual (real world) dimensions of those objects (like furniture and buildings).

Worked example 2: Using the number scale to estimate distance

Study the school map given below and answer the questions that follow:

  1. Calculate the following real dimensions of the sports field in metres:
    1. length
    2. width
  2. Calculate the length of the science classroom block in metres
  3. Zuki walks from the tuckshop to his maths classroom, along the blue dotted line shown. Measure how far he walked in metres.
    1. Use your ruler to measure the width of the sports field on the map. It is \(\text{5}\) \(\text{cm}\) wide. Now use the number scale \(\text{1}\) : \(\text{500}\) to determine the actual width of the field: \(\text{5}\) \(\text{cm}\) \(\times\) \(\text{500}\) = \(\text{2 500}\) \(\text{cm}\) (multiply your scaled measurement by the “real” number in the scale ratio) \(\text{2 500}\) \(\text{cm}\) \(\div\) \(\text{100}\) = \(\text{25}\) \(\text{m}\) The field is \(\text{25}\) \(\text{m}\) wide
    2. On the map, the field is \(\text{10}\) \(\text{cm}\) long. \(\text{10}\) \(\times\) \(\text{500}\) = \(\text{5 000}\) \(\text{cm}\) \(\text{5 000}\) \(\text{cm}\) \(\div\) \(\text{100}\) = \(\text{50}\) \(\text{m}\) The field is \(\text{50}\) \(\text{m}\) long
  1. On the map, the science classroom building is \(\text{5}\) \(\text{cm}\) long. \(\text{5}\) \(\text{cm}\) \(\times\) \(\text{500}\) = \(\text{2 500}\) \(\text{cm}\) \(\text{2 500}\) \(\text{cm}\) \(\div\) \(\text{100}\) = \(\text{25}\) \(\text{m}\) The science classrooms are \(\text{25}\) \(\text{m}\) long
  2. The blue dotted line is \(\text{6}\) \(\text{cm}\) long on the map. \(\text{6}\) \(\times\) \(\text{500}\) = \(\text{3 000}\) \(\text{cm}\) \(\text{3 000}\) \(\text{cm}\) \(\div\) \(\text{1 000}\) = \(\text{30}\) \(\text{m}\) Zuki walked \(\text{30}\) \(\text{m}\) from the tuckshop to his maths classroom.

Understanding the advantages and disadvantages of each scale (EMG4T)

By now you should understand how to use number and bar scales to measure real dimensions and distance on the ground when given a scale map. What happens if you resize a map though (for example you may want to make small photocopies of a map of your school, to hand out for an event taking place)? In the next example we will explore the effects on the number and bar scales when we resize maps.

Worked example 3: Resizing and accuracy

Diagram 1

Diagram 2

Diagram 3

Diagram 4

  1. Measure the width of the school bag in Diagram 1 and use the scale to calculate the width of the school bag.
  2. Measure the school bag in Diagram 2 and use the scale to calculate the width of the school bag.
  3. What do you notice about the answers for 1. and 2.?
  4. Measure the width of the school bag in Diagram 3 and use the scale to calculate the width of the school bag.
  5. Measure the school bag in Diagram 4 and use the scale to calculate the width of the school bag.
  6. What do you notice about the answers for 4. and 5.?
  7. Write a sentence to explain what you have learnt as a result of your calculations.
  1. The measured width on the diagram is \(\text{3}\) \(\text{cm}\). Therefore \(\text{3}\) \(\text{cm}\) \(\times\) \(\text{15}\) = \(\text{45}\) \(\text{cm}\). So the bag is \(\text{45}\) \(\text{cm}\) wide.
  2. The measured width on the diagram is \(\text{5}\) \(\text{cm}\). Therefore \(\text{5}\) \(\text{cm}\) \(\times\) \(\text{15}\) = \(\text{75}\) \(\text{cm}\). So the bag is now \(\text{75}\) \(\text{cm}\) wide!
  3. These answers are very different. Which is correct? Is the bag \(\text{45}\) \(\text{cm}\) wide or \(\text{75}\) \(\text{cm}\) wide?
  4. \(\text{1}\) segment = \(\text{1}\) \(\text{cm}\) long. The bag is \(\text{3}\) \(\text{cm}\) wide on the diagram, therefore \(\text{3}\) \(\times\) \(\text{15}\) \(\text{cm}\) = \(\text{45}\) \(\text{cm}\).
  5. \(\text{1}\) segment = \(\text{1,5}\) \(\text{cm}\) long. The bag is \(\text{4,5}\) \(\text{cm}\) wide on the diagram. \(\text{4,5}\) \(\div\) \(\text{1,5}\) = \(\text{3}\) segments. \(\text{3}\) segments \(\times\) \(\text{15}\) \(\text{cm}\) = \(\text{45}\) \(\text{cm}\)
  6. The answers for Questions \(\text{4}\) and \(\text{5}\) are the same!
  7. When resizing scale diagrams using the number scale, we have to change the scale in order for it to remain accurate. (In the bigger diagram we would need to number scale to be \(\text{1}\) : \(\text{9}\) for the width of the bag to be \(\text{45}\) \(\text{cm}\)). When resizing diagrams using the bar scale, the length of the segments increases proportionally to the diagram, therefore the resized bar scale is also accurate and will give us the same answer.

If we resize a map that has a number scale on it, the number scale becomes incorrect. If a map is \(\text{10}\) \(\text{cm}\) wide when printed, and the number scale is \(\text{1}\) : \(\text{10}\) then \(\text{1}\) \(\text{cm}\) on the map represents \(\text{10}\) \(\text{cm}\) on the ground. However, if we reprint the map larger, and it is now \(\text{15}\) \(\text{cm}\) wide, our scale will still be \(\text{1}\) : \(\text{10}\) according to the map, but now \(\text{1,5}\) \(\text{cm}\) represents \(\text{10}\) \(\text{cm}\) on the ground (\(\text{1,5}\) \(\times\) \(\text{10}\) = \(\text{15}\) \(\text{cm}\) = width of map) so the answers to any scale calculations will now be wrong. When resizing maps that use the number scale, it is important to know that the scale changes with the map. This is a disadvantage to using the number scale.

If we resize a map that has a bar scale on it, the size of the bar scale will be resized with the map, and it will therefore remain accurate. This is an advantage to using the bar scale.

An advantage of the number scale is that we only have to measure one distance (we don't have to measure the length of one bar segment) and our calculations are usually fairly simple as a result. A disadvantage to using the bar scale is that we have to measure the length of one segment and measure the distance on the map, and our calculations can be more complicated because we have to calculate how many segments fit into the distance measured on the map.

Drawing a scaled map when given real (actual) dimensions (EMG4V)

We have learnt how to determine actual measurements when given a map and a scale. In this section we will look at the reverse process - how to determine scaled measurements when given actual dimensions, and draw an accurate two dimensional map. Remember that a scale drawing is exactly the same shape as the real (actual) object, just drawn smaller. In the next worked example we will look at how to draw a simple scaled map of a room.

In order to draw a map you need two pieces of information. Firstly you need to know the actual measurements of everything that has to go onto the map. Secondly you need to know what scale you have to use. The scale will depend on the original measurements, how much detail the map has to show and the size of the map. If you want to draw a map, or plan, of a room in your house on a sheet of A4 paper and include detail of the furniture you would not use a scale of \(\text{1}\): \(\text{10 000}\) (this scale means that \(\text{1}\) \(\text{cm}\) in real life is equal to \(\text{10 000}\) \(\text{cm}\) or \(\text{1}\) \(\text{km}\) in real life).

In Grade \(\text{10}\) the scale will be given to you.

Worked example 4: Drawing scaled maps

Draw a scaled map of a room that has real dimensions \(\text{3}\) \(\text{m}\) by \(\text{4,5}\) \(\text{m}\). Use a number scale of \(\text{1}\) : \(\text{50}\).

The scale of \(\text{1}\): \(\text{50}\) means that \(\text{1}\) unit on your drawing will represent \(\text{50}\) units in real life so \(\text{1}\) \(\text{cm}\) on your drawing will represent \(\text{50}\) \(\text{cm}\) in real life.

  • The width of the room is \(\text{3}\) \(\text{m}\).

    • Convert \(\text{3}\) \(\text{m}\) to cm: \(\text{3}\) \(\text{m}\) \(\times\) \(\text{100}\) = \(\text{300}\) \(\text{cm}\)
    • Use the scale to calculate the scaled width on the map: \(\text{300}\) \(\text{cm}\) \(\div\) \(\text{50}\) \(\text{cm}\) = \(\text{6}\) \(\text{cm}\) (Divide the actual, real measurement of the room by the 'real number' from the scale)
  • The length of the room is \(\text{4,5}\) \(\text{m}\).

    • Convert \(\text{4,5}\) \(\text{m}\) to cm: \(\text{4,5}\) \(\times\) \(\text{100}\) = \(\text{450}\) \(\text{cm}\)
    • Use the scale to calculate the scaled length on the map: \(\text{450}\) \(\text{cm}\) \(\div\) \(\text{50}\) \(\text{cm}\) = \(\text{9}\) \(\text{cm}\)
  • The scaled measurements are \(\text{6}\) \(\text{cm}\) and \(\text{9}\) \(\text{cm}\). We can now draw this on our plan. Don't forget to include the scale on your map!

Worked example 5: Drawing a scaled map

In this worked example we will add some furniture to the room in the previous example.

The room has the same dimensions (\(\text{3}\) \(\text{m}\) \(\times\) \(\text{4,5}\) \(\text{m}\)) and the scale to be used is still \(\text{1}\) : \(\text{50}\).

Draw the following items using the dimensions provided:

  1. A couch \(\text{2}\) \(\text{m}\) \(\times\) \(\text{1,2}\) \(\text{m}\)
  2. A window \(\text{2}\) \(\text{m}\) long
  3. A table \(\text{1,5}\) \(\text{m}\) wide and \(\text{2}\) \(\text{m}\) long.

You may arrange the furniture in the room in any way which you think is sensible.

The scale of \(\text{1}\): \(\text{50}\) means that \(\text{1}\) unit on your drawing will represent \(\text{50}\) units in real life so \(\text{1}\) \(\text{cm}\) on your drawing will represent \(\text{50}\) \(\text{cm}\) in real life.

The scaled dimensions of the room are the same as in the previous worked example: \(\text{6}\) \(\text{cm}\) \(\times\) \(\text{9}\) \(\text{cm}\).

  1. The width of the couch is \(\text{1,2}\) \(\text{m}\). \(\text{1,2}\) \(\text{m}\) = \(\text{120}\) \(\text{cm}\) \(\text{120}\) \(\text{cm}\) \(\div\) \(\text{50}\) = \(\text{2,4}\) \(\text{cm}\) The length of the couch is \(\text{2}\) \(\text{m}\) \(\text{2}\) \(\text{m}\) = \(\text{200}\) \(\text{cm}\) \(\text{200}\) \(\div\) \(\text{50}\) = \(\text{4}\) \(\text{cm}\). So the scaled dimensions of the couch are \(\text{2,4}\) \(\text{cm}\) and \(\text{4}\) \(\text{cm}\)
  2. The length of the window is \(\text{2}\) \(\text{m}\) \(\text{2}\) \(\text{m}\) = \(\text{200}\) \(\text{cm}\) \(\text{200}\) \(\div\) \(\text{50}\) = \(\text{4}\) \(\text{cm}\). So the scaled dimension for the length of the window is \(\text{2}\) \(\text{cm}\)
  3. The width of the table is \(\text{1}\) \(\text{m}\). \(\text{1}\) \(\text{m}\) = \(\text{100}\) \(\text{cm}\) \(\text{100}\) \(\text{cm}\) \(\div\) \(\text{50}\) = \(\text{2}\) \(\text{cm}\) The length of the table is \(\text{1,5}\) \(\text{m}\) \(\text{1,5}\) \(\text{m}\) = \(\text{150}\) \(\text{cm}\) \(\text{150}\) \(\div\) \(\text{50}\) = \(\text{3}\) \(\text{cm}\). So the scaled dimensions of the table are \(\text{2}\) \(\text{cm}\) and \(\text{3}\) \(\text{cm}\)

Drawing a scaled map

Exercise 6.2

The bedroom in the picture is \(\text{3,5}\) \(\text{m}\) by \(\text{4}\) \(\text{m}\). It has a standard sized single bed of \(\text{92}\) \(\text{cm}\) by \(\text{188}\) \(\text{cm}\). The bedside table is \(\text{400}\) \(\text{mm}\) square. Draw a floor plan to show the layout of the room. Use the number scale \(\text{1}\) : \(\text{50}\).

Room real measurements:

width \(\text{3,5}\) \(\text{m}\) = \(\text{350}\) \(\text{cm}\)

length \(\text{4}\) \(\text{m}\) = \(\text{400}\) \(\text{cm}\)

Scale drawing:

\(\text{350}\) \(\div\) \(\text{50}\) = \(\text{7}\) \(\text{cm}\)

\(\text{400}\) \(\div\) \(\text{40}\) = \(\text{8}\) \(\text{cm}\)

Bed real measurements:

Width = \(\text{92}\) \(\text{cm}\)

Length = \(\text{188}\) \(\text{cm}\)

Scale drawing:

\(\text{92}\) \(\text{cm}\) \(\div\) \(\text{50}\) = \(\text{1,84}\) \(\text{cm}\)

\(\text{188}\) \(\text{cm}\) \(\div\) \(\text{50}\) = \(\text{3,76}\) \(\text{cm}\)

Bedside table real measurements:

\(\text{400}\) \(\text{mm}\)

\(\text{400}\) \(\text{mm}\) \(\div\) \(\text{50}\) = \(\text{8}\) \(\text{mm}\)

Scale drawing:

drawing scaled maps

Exercise 6.3

Divide into groups in order to measure and draw an accurate, scaled floor plan of your classroom. Use a scale of \(\text{1}\) : \(\text{50}\). You will need to measure all the large objects (e.g. desks, windows, the blackboard) in the classroom, calculate what their scaled dimensions will be and then draw them carefully on your floor plan. Can you think of a different or better way to arrange the furniture in your classroom?

For this activity, we suggest providing square grid paper for the learners' to use, to make the drawing process easier. You will also need to provide measuring tapes for the learners to use to measure the actual dimensions of the furniture in the classroom. If the classroom is very large or has lots of furniture, the activity can be simplified by assigning a section of the classroom to each group, so that they have less to measure and draw.

learner-dependent answer